The present invention relates to a liquid crystal display device, and more particularly to a lateral-electric-field-type liquid crystal display device which can enhance a display numerical aperture.
As a display device for various kinds of personal digital assistants and television receiver sets, a display device which uses a so-called flat-type display panel as represented by a liquid crystal display device has been a mainstream. Further, as the liquid crystal display device, an active-method-type liquid crystal display device has been popularly used. The active-method-type liquid crystal display device generally uses a thin film transistor as a drive element of a pixel and hence, hereinafter, the display device which adopts the thin film transistor as the drive element is explained as an example. As one type of such a liquid crystal display device, there has been known a lateral-electric-field-type liquid crystal display device which is referred to IPS (in-plane-switching) type display device.
FIG. 9 is a plan view for explaining an example of the base pixel structure of the thin film transistor of the IPS type liquid crystal display device. Further, FIG. 10 is a cross-sectional view of the display device which also includes a color filter substrate taken along a line D-D′ in FIG. 9. The planer constitution of the pixel of the IPS type liquid crystal display device is, as shown in FIG. 9, formed in the inside of a region which is surrounded by two gate lines GL and two data lines DL. A thin film transistor TFT is formed on a portion of the region, (pixel region). The thin film transistor TFT has a drain (or a source) electrode SD2 thereof connected to the data line DL, has a gate electrode GT thereof connected to the gate line GL, and has a source (drain) electrode SD1 connected to a pixel electrode PX through a contact hole CH. Here, although the drain electrode and the source electrode are exchanged from each other during an operation, the explanation is made hereinafter with respect to a case in which the thin film transistor TFT includes the source electrode SD1 and the drain electrode SD2.
As shown in FIG. 10, the cross-sectional structure of the pixel forms the thin film transistor TFT which is constituted of a semiconductor layer (silicon semiconductor) SI, a second insulation film INS2, the gate electrode GT, a third insulation film INS3, the source electrode SD1 and the drain electrode SD2 on a first insulation film INS1 which is formed on a main surface of one substrate (a thin film transistor substrate, hereinafter, a TFT substrate) SUB1 which is preferably made of glass. Here, the scanning lines GL shown in FIG. 9 are formed on the same layer as the gate electrodes GT, the data lines DL are formed on the third insulation film INS3, and the source electrodes SD1 and the drain electrodes SD2 are formed on the same layer as the data lines DL. The source electrodes SD1 and the drain electrodes SD2 are connected to the semiconductor layers SI via the contact holes which are formed in the second insulation film INS2 at the time of forming the source electrodes SD1, drain electrodes SD2 as films.
A fourth insulation film INS4 which constitutes a protective film (passivation film) is formed in a state that the fourth insulation film INS4 covers the source electrode SD1, the drain electrode SD2 and the data lines DL. Here, a counter electrode CT is formed in a spreading manner on the fourth insulation film INS4 in a state that a contact electrode CT covers a most portion of the pixel region, and a contact hole CH which reaches the source electrode SD1 is formed in the fourth insulation film INS4. Further, a fifth insulation film INS5 is formed in a state that the fifth insulation film INS5 covers the counter electrode CT.
The pixel electrode PX is formed on the fifth insulation film INS5 in a comb-teeth shape, and one end of the pixel electrode PX is connected to the source electrode SD1 via the contact hole CH. Then, an orientation film ORI1 is formed in a state that the orientation film covers a topmost surface of the pixel electrode PX.
On a main surface of another substrate (color filter substrate, hereinafter, referred to as a CF substrate) SUB2 which is preferably made of glass, color filters CF which are defined from each other by a black matrix BM are formed, and an orientation film ORI2 is formed on a topmost surface of the substrate SUB2. The current display devices mostly adopt a full color display. In this full color display, basically, unit pixels (sub pixels) of three colors consisting of red (R), green (G), and blue (B) constitute one color pixel.
In the IPS type liquid crystal display device, a liquid crystal LC is sealed in the inside of a space between the orientation film ORI1 of the TFT substrate SUB1 and the orientation film ORI2 of the CF substrate SUB2. The liquid crystal LC which is driven by the thin film transistor TFT is rotated by a component of an electrical field E parallel to a surface of the substrate which is generated between the pixel electrode PX and the counter electrode CT in the inside of the surface in which the orientation direction of the liquid crystal LC is parallel to the surface of the substrate and hence, the lighting and non-lighting of the pixel can be controlled. As a document which discloses such an IPS-type liquid crystal display device, International Publication WO 01/018597 can be named.